In recent years, thermal transfer systems have been developed to obtain printed images from pictures which have been generated electronically from a digital record of the original scene. The digital record can be generated directly from an electronic camera such as a video camera, or indirectly, by scanning a photographic print or negative. In essence, a digital photographic record consists of a digital number, or signal, stored on a magnetic or optical medium, that can be converted into an electrical signal whose strength is proportional to the optical density of a given pixel of the image. The digital record may be of one color (monochrome, or black and white), or may consist of records of color separations, commonly as the scene would be viewed through red, green and blue filters. The electrical signals corresponding to the digital record may then be transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element having an image-receiving layer. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 by Brownstein entitled "Apparatus and Method for Controlling A Thermal Printer Apparatus," issued Nov. 4, 1986.
A common problem with prints made by thermal dye transfer is that of re-transfer from the image-receiving layer, wherein the dyes of the image may diffuse into sheets of paper or plastic which are stored in contact with the thermal prints. This kind of problem can sometimes be seen when a stack of thermal prints is stored; after storage part of the image dyes are found to be transferred to the back of the print above in the stack.
Another kind of thermally printed image, or pattern, is disclosed in U.S. Pat. No. 4,923,860 by Simons entitled "Method of Making a Color Filter Array Using Light Flash," issued May 8, 1990. This patent describes the preparation of a color filter array to be used in a liquid crystal display device.
Liquid crystal display devices are known for digital display in electronic calculators, clocks, household appliances, audio equipment, etc. Liquid crystal displays are being developed to replace cathode ray tube technology for display terminals. Liquid crystal displays occupy a smaller volume than cathode ray tube devices with the same screen area. Also, they are lighter than cathode ray tubes, and are therefore useful in portable device displays such as lap-top computers. In addition, liquid crystal display devices usually have lower power and lower voltage requirements than corresponding cathode ray tube devices.
One commercially available type of color filter array element that has been used in liquid crystal display devices for color display capability is a transparent support having a gelatin layer thereon which contains dyes having the additive primary colors red, green and blue in a mosaic pattern obtained by a photolithographic technique. To prepare such a color filter array element a gelatin layer is sensitized, exposed to a mask for one of the colors of the mosaic pattern, developed to harden the gelatin in the exposed areas, and washed to remove the unexposed (uncrosslinked) gelatin, thus producing a pattern of gelatin which is then dyed with dye of the desired color. The element is then recoated and the above steps are repeated to obtain the other two colors. Misalignment or improper deposition of color materials may occur during any of these operations. For a more complete description of this process see in U.S. Pat. No. 4,081,277.
Color liquid crystal display devices generally include two spaced glass panels which define a sealed cavity that is filled with a liquid crystal material. For actively-driven devices, a transparent electrode is formed on one of the glass panels, which electrode may be patterned or not, while individually addressable electrodes are formed on the other of the glass panels. Each of the individual electrodes has a surface area corresponding to the area of one picture element, or pixel. If the device is to have color capability, each pixel must be aligned with a color area, e.g. red, green, or, blue, of a color filter array. Depending on the image to be displayed, one or more of the pixel electrodes is energized during display operation to allow full light, no light, or partial light to be transmitted through the color filter area associated with that pixel. The image perceived by a user is a blending of colors formed by the transmission of light through adjacent color filter areas. In the display of high quality images, it is important that the pixel elements be of pure color, with the correct hue, high saturation of color, and with minimal unwanted absorption of other colors. When dyes are used as the colorants, subsequent processing steps involving heat can cause dye migration, or smearing, which leads to color contamination of adjacent color pixels.